In spite of what has been widely reported, The Los Angeles Times says Tupac did not appear as a hologram at Coachella:

Tupac Shakur image at Coachella: In the April 17 and April 18 Calendar sections, articles about the Coachella Music and Arts Festival referred to the late rapper Tupac Shakur having appeared as a hologram during a performance by Snoop Dogg and Dr. Dre. The image was two-dimensional and thus not a hologram.

That is exactly correct, and well worth pointing out. It was a reflection – albeit a brilliantly created reflection. “Hologram” means three-dimensional: you can walk behind it and see portions that are hidden when viewed from the front. This was not the case with the Tupac show.